Providing authenticity and security for photo identification and providing identifying information in consumer products are two of the most common applications of encoded markings such as bar codes. In many implementations, bar codes are black and white bars in which the information is encoded in the widths of the bars. The bar code is read with an optical reader comprised of a source that emits radiation in a range of wavelengths, means of scanning the radiation across the bar code and a detector that receives the reflected radiation. The information can be decoded from the electrical signal produced by the detector since the reflectance from the black bars is significantly different than that from the white bars.
While such an encoding method is commonly used, for a small object the bar code occupies a significant portion of the object and it detracts from the esthetics of the product. Also, a black and white bar code is susceptible to forgery once the code is deciphered or if the bar code adheres to a known standard.
In response to the first of these shortcomings, bar codes that are essentially non-visible and which can be read with infrared radiation have been developed. To implement the non-visible bar codes, the material deposited on the object or receiving medium must be invisible under radiation in the visible range but detectable under infrared radiation. Proposed materials included organic dyes such as cyanine based dyes and naphthoquinone dyes (U.S. Pat. No. 5,911,921) and inorganic materials such as Ytterbium phosphate (U.S. Pat. No. 5,911,921). The organic dyes are not stable in harsh environments and have some selective absorption in the visible range. The inorganic materials can be expensive to manufacture although the Ytterbium phosphate powder disclosed and claimed in U.S. Pat. No. 5,911,921 could represent a lower cost solution.
However, any bar code implemented with a material having high absorption in near infrared does not pose a solution to the problem of preventing forgeries since the use of an infrared scope or viewer will make the bar code detectable and the bar code could be counterfeited using infrared absorbers.
One solution, disclosed in U.S. Pat. No. 5,760,384, is to use a material that absorbs infrared radiation within a narrow band of wavelengths. One embodiment disclosed in U.S. Pat. No. 5,760,384 is a phthalocyanine which could exhibit instability in harsh environments.
An alternate approach to solving the problem of preventing forgeries is to detect the forgeries. U.S. Pat. No. 5,760,384 also discloses an apparatus and method for judging whether a bar code, comprising a material that absorbs infrared radiation within a narrow band of wavelengths, is real or a forgery. The disclosed method comprises detecting the reflectance at the peak absorption wavelength and at another neighboring wavelength away from the peak absorption wavelength.
U.S. Pat. No. 5,336,252 proposes another approach to preventing forgeries of infrared bar codes. In U.S. Pat. No. 5,336,252 the infrared bar code is covered with a layer of an ink that has high absorption in the visible and is transparent to the infrared. One embodiment is an ink prepared by mixing and dispersing a white pigment, such as titanium oxide or zinc oxide, with an extender such as calcium carbonate. While this approach will make it more difficult to forge the bar code, once the presence of the barcode is detected, it is not forgery proof.
All the above inventions relate to encoding the information in a marking created by depositing onto a medium a material that is capable of strongly absorbing radiation over a range of infrared wavelengths and substantially non-absorbing in the visible wavelengths. The necessity of strong absorption is derived from the decoding requirement that the reflectance, in the infrared range of wavelengths, from the infrared absorbing material is significantly different than that from the medium. This requirement will ensure that the electrical signal from the detector that receives the reflected infrared radiation is sufficiently distinct from the background noise so that it can be reliably decoded.
The large group of materials that are capable of mildly absorbing radiation over a range of infrared wavelengths and substantially non-absorbing in the visible wavelengths, such as most synthetic polymers (for example, polystyrene, Polyethylene terethalate), do not find application in the encoding of information as markings on a medium since the resulting electrical signal would not be sufficiently distinct from the background noise to be reliably decoded if read with the optical reader previously described. These large group of materials includes many low cost materials that would be attractive candidate materials if the information encoded in marks could be decoded.
The object of the present invention is to present methods for the use of lower cost, stable materials to encode information onto or on a base medium, and method and apparatus for reading the encoded information while retaining robustness to forgeries.